def pr(x):
    for i in range(2,int(x**0.5)+1):
        if x % i == 0:
            return False
            break
    return True


prime = []
for x in range(2,1000):
    if pr(x):
        prime.append(x)
# print(prime)

x = int(input())
if pr(x):
    print(x)
else:
    li = []
    x = str(x)
    li.append(int(x[0]))
    li.append(int(x[0:2]))
    li.append(int(x[0:3]))
    li.append(int(x[1:4]))
    li.append(int(x[2:4]))
    li.append(int(x[3:4]))
    li.append(int(x[1]))
    li.append(int(x[2]))
    li.append(int(x[3]))
    li.append(int(x[1]))
    li.append(int(x[1:3]))
    li.append(int(x[2:4]))
    
    maxprime = 0
    # print(li)
    for y in li:
        if pr(y):
            maxprime = max(maxprime, y)
    print(maxprime)


